A shift kit will make it shift harder and crisper than standard and extend the life of the auto because its the sloppy shifts that stock autos have that create strain and wear. But note that it will put extra strain on your tailshaft and diff (kaboom).
You'd be better off converting to manual, unless you really want to stick with the auto. The manual with a good clutch has less driveline loss and is generally alot more pokier. Same goes with most cars though I guess. I know vn manuals are little monsters